Gel-like fuel for fuel cell

ABSTRACT

According to one embodiment, a gel-like fuel for a fuel cell includes a network macromolecular compound formed by cross-linking a macromolecular compound having at least one group selected from an OH group and a COOH group, or a C═N bond with a cross-linking agent, a liquid fuel component incorporated into the network macromolecular compound, and a co-catalyst incorporated into the network macromolecular compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2008-078989, filed Mar. 25, 2008, theentire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

One embodiment of the present invention relates to a gel-like fuel usedfor fuel cells.

2. Description of the Related Art

Fuel cells using methanol or diethyl ether as the liquid fuel each havethe electromotive section which comprises an anode (fuel electrode) towhich the above fuel is fed, a cathode (air electrode) to which anoxidizer (oxygen or air) is fed, and a polymer electrolyte membraneinterposed between the anode and the cathode. The anode comprises acatalyst layer, which is in contact with the polymer electrolytemembrane, and a diffusion layer such as carbon paper laminated on thecatalyst layer.

In the above fuel cell, methanol or diethyl ether to be used as theliquid fuel is volatile and flammable, thus it is necessary to handlethese fuels with care.

From the above fact, Jpn. Pat. Appln. KOKAI Publication No. 2007-242367discloses technologies that improve the handling ability of liquid fuelby using a microcapsule to include the liquid fuel to thereby put thefuel into a solid form.

However, the microcapsules included within the fuel cell reduce thespeed of the methanol that needs to reach the catalyst layer of the fuelelectrode. As a result, the oxidation reaction necessary for powergeneration is reduced, causing a reduction in output.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A general architecture that implements the various feature of theinvention will now be described with reference to the drawings. Thedrawings and the associated descriptions are provided to illustrateembodiments of the invention and not to limit the scope of theinvention.

FIG. 1 is a view showing the current-voltage characteristic of a unitcell when each fuel obtained in Examples 1 to 4 and Comparative Example1 is used; and

FIG. 2 is a view showing the current-voltage characteristic of a unitcell when each fuel obtained in Examples 5 to 8 and Comparative Example1 is used.

DETAILED DESCRIPTION

Various embodiments according to the invention will be describedhereinafter. In general, according to one embodiment of the invention,there is provided a gel-like fuel for a fuel cell comprising: a networkmacromolecular compound formed by cross-linking a macromolecularcompound having at least one group selected from an OH group and a COOHgroup, or a C═N bond with a cross-linking agent; a liquid fuel componentincorporated into the network macromolecular compound; and a co-catalystincorporated into the network macromolecular compound.

A typical fuel cell comprises an anode (fuel electrode) to which theabove gel-like fuel is fed, a cathode (air electrode) to which anoxidizer (oxygen or air) is fed, and a polymer electrolyte membraneinterposed between the anode and the cathode. The anode comprises acatalyst layer, which is in contact with the polymer electrolytemembrane, and a diffusion layer such as carbon paper formed on thecatalyst layer. The cathode comprises a catalyst layer, which is incontact with the polymer electrolyte membrane, and a diffusion layersuch as carbon paper formed on the catalyst layer.

The macromolecular compound is preferably an aliphatic macromolecularcompound containing at least one group selected from an OH group andCOOH group or a C═N bond. Examples of the aliphatic macromolecularcompound having an OH group include a polyvinyl alcohol and polyethyleneglycol. Examples of the aliphatic macromolecular compound having a COOHgroup include a polyacrylic acid. The aliphatic macromolecular compoundhaving an OH group and COOH group is for example, a polyalginic acid.The aliphatic macromolecular compound having a C═N bond is for example,a polyethyleneimine.

The cross-linking agent is preferably, for example, an isocyanate inwhich a NCO group is bound with a long-chain alkyl group.

Examples of the fuel component include lower alcohols such as methanoland ethanol or ethers such as dimethyl ether.

The co-catalyst has the ability to activate an oxidation reaction when afuel component in the gel-like fuel reaches the fuel electrode of thefuel cell. Such a co-catalyst is for example, a porphyrin.

The liquid fuel component is preferably incorporated into the networkmacromolecular compound in an amount of 15 to 99.5% by weight based onthe total amount of the network macromolecular compound and the liquidfuel component. A gel-like fuel for a fuel cell in which the liquid fuelcomponent is incorporated into the network macromolecular compound onsuch an amount can better promote the speed required for the liquid fuelcomponent to reach the catalyst layer of the fuel electrode, whenapplied to a fuel cell.

The co-catalyst is preferably incorporated into the networkmacromolecular compound in an amount of 0.001 to 30% by weight based onthe total amount of the network macromolecular compound and theco-catalyst.

The gel-like fuel according to the embodiment preferably has a shearingviscosity of 100 to 1500000 dyne·s/cm². The shearing viscosity of thegel-like fuel primarily is compatible, in the degree of cross-linking,with the network macromolecular compound. When the shearing viscosity isdesigned to be less than 100 dyne·s/cm², there is a fear that thenetwork macromolecular compound is deteriorated in the ability toincorporate the liquid fuel component. On the other hand, the shearingviscosity of the gel-like fuel exceeds 1500000 dyne·s/cm², the networkmacromolecular compound is deteriorated in the ability to release theliquid fuel component and there is therefore a fear of a decrease in thespeed required for the network macromolecular compound to reach thecatalyst layer of the fuel electrode, when applied to a fuel cell. Theshearing viscosity of the gel-like fuel for a fuel cell is morepreferably 1000 to 300000 dyne·s/cm².

The gel-like fuel for a fuel cell according to the embodiment may beproduced, for example, by the following method.

First, a reaction container is charged with the macromolecular compoundand a cross-linking agent together with a solvent and the mixture isstirred and mixed under heating to allow the macromolecular compound toundergo a cross-linking reaction. Subsequently, the reaction product isput in the heated state to distill solvents under reduced pressure,thereby obtaining a network macromolecular compound. In the case of, forexample, an aliphatic macromolecular compound having an OH group, thisOH group reacts with the cross-linking agent to cross-link, therebyproducing a network macromolecular compound. Then, the liquid fuelcomponent is gradually added to the network macromolecular compound withstirring, and after all the liquid fuel component is added, the stirringis further continued and stopped when all of the gel is made uniform, tothereby produce a gel-like fuel for a fuel cell. In this case, theco-catalyst may be added either in the raw material stage of preparing anetwork macromolecular compound or before the liquid fuel component isadded after the network macromolecular compound is prepared.

The macromolecular compound and the cross-linking agent are preferablycompounded in an amount of 0.3 to 60% by weight and in an amount of 0.01to 20% by weight respectively. The network macromolecular compound whichis cross-linked between macromolecular compound and the cross-linkingagent in such a compounding ratio to form a network is well balancedbetween the ability to incorporate the liquid fuel component and theability to release the liquid fuel component.

The gel-like fuel for a fuel cell according to the embodiment asmentioned above has the following ability and effect when applied to afuel cell.

(1) The gel-like fuel for a fuel cell comprises a network macromolecularcompound formed by cross-linking a macromolecular compound having atleast one group selected from an OH group and a COOH group, or a C═Nbond with a cross-linking agent, a liquid fuel component incorporatedinto the network macromolecular compound, and a co-catalyst incorporatedinto the network macromolecular compound. Therefore, when the fuel isfed to the catalyst layer of the fuel electrode in the fuel cell, theliquid fuel component is rapidly released from the networkmacromolecular compound and reaches the catalyst layer. Specifically,the speed required for the liquid fuel component to reach the catalystlayer can be increased. As a result, the oxidation reaction necessaryfor power generation can sufficiently proceed in the catalyst layer,making it possible to improve the output density of the fuel cell.

(2) The crossover phenomenon, in which an unreacted liquid fuelcomponent retained in the catalyst layer diffuses into the polymerelectrolyte membrane to reach the oxidizing electrode, can be reduced.Specifically, a proper amount of the liquid fuel component is releasedfrom the network macromolecular compound and reaches the catalyst layerof the fuel electrode, where the oxidation reaction of the fuel canproceed. Therefore, such a phenomenon that an unreacted liquid fuelcomponent is retained in the catalyst layer can be further limited.Also, the co-catalyst is likewise released from the networkmacromolecular compound and reaches the catalyst layer to activate theliquid fuel component which has already reached there to promote theoxidation reaction. Therefore, such a phenomenon that an unreactedliquid fuel component is retained in the catalyst layer can be evenfurther limited. Since such a phenomenon that an unreacted liquid fuelcomponent is retained in the catalyst layer can be limited, the abovecrossover phenomenon can be reduced.

(3) Since the retention of an unreacted liquid fuel component can belimited as explained in the above (2), the generation of heat issuppressed, and the efficiency of power generation can be improved.

(4) The damage of devices mounted on the fuel cell which are caused bythe leakage of the liquid fuel component can be further reduced than inthe case of singly using a liquid fuel.

(5) A reduction in the size of the fuel cell can be attained by theeffects of the above (1) to (3).

The present invention will be explained in more detail by way ofexamples. However, these examples are not intended to be limiting of thepresent invention.

EXAMPLE 1 Preparation of a Network Macromolecular Compound

A reaction container obtained by equipping a three-neck flask having acircular bottom with a homogenizer (trade name: Homogenizer PH91,manufactured by MST (Corp.), a Liebig condenser and a dropping funnelwas soaked in a silicon oil bath.

30 parts by weight of a polyvinyl alcohol (manufactured by AldrichCorporation, weight average molecular weight: 31000 to 50000) used as amacromolecular compound, 5 parts by weight of methylene diisocyanate(manufactured by Aldrich Corporation) used as a cross-linking agent and0.5 parts by weight of porphyrin used as a co-catalyst were added in64.5 parts by weight of 1,2-dichloroethylene (manufactured by AldrichCorporation), to prepare a solution. This solution was poured into thereaction container and stirred at 550° C. at 3000 rpm by using ahomogenizer for 2 hours. The mixture in the reaction container waspoured into an eggplant-shape flask and then, a rotary evaporator (tradename: RI-210, manufactured by Shibata Rika Kiki)) was set to the flaskto distill 1,2-dichloroethylene which was a solvent component underreduced pressure in the hot-water bath to prepare a porphyrin-dispersednetwork macromolecular compound. The obtained network macromolecularcompound has a number average molecular weight of 100000.

[Production of a Fuel]

10 g of the obtained porphyrin-dispersed network macromolecular compoundwas poured into a 300 mL beaker and then 90 g of methanol was graduallyadded with stirring the network macromolecular compound in the beaker bya homogenizer (trade name: Homogenizer PH91, manufactured by MST(Corp.)). After the above methanol was all added, the stirring wascontinued for 30 minutes to produce a gel-like fuel for a fuel cellwhich was uniformed as a whole.

EXAMPLES 2 TO 8

Gel-like fuels for a fuel cell were produced in the same manner as inExample 1 except that the materials shown in the following Table 1 wereused as the macromolecular compound, cross-linking agent, co-catalystand solvent used to prepare a co-catalyst dispersed networkmacromolecular compound in each amount shown in Table 1, and the networkmacromolecular compound and methanol used in the production of thegel-like fuel were formulated in the ratios shown in Table 1. Thepolyethyleneimine, polyalginic acid and polyacrylic acid, which were themacromolecular compounds, had weight average molecular weights of 2000to 350000, 3000 to 450000 and 2500 to 200000 respectively.

The shearing viscosity of each of the gel-like fuels obtained inExamples 1 to 8 is described in Table 1 below.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 Composition of Polyvinyl alcohol 20 — — — 20 — — —Porphyrin-dispersed Polyethylene imine — 25 — — — 25 — — networkPolyalginic acid — — 30 — — — 30 — macromolecular Polyacrylic acid — — —35 — — — 35 compound Methylene diisocyanate 5 10 15 20 — — — — (part byweight) Toluylene diisocyanate — — — — 5 10 15 18 Porphyrin 0.5 0.5 0.50.5 1.0 1.0 1.0 1.0 1,2-dichloroethylene 74.5 64.5 54.5 44.5 — — — —N,N-dimethylformamide — — — — 74 64 54 46 Number average molecularweight of the 100000 125000 140000 160000 120000 140000 170000 180000Porphyrin-dispersed network macromolecular compound FuelPorphyrin-dispersed 10 15 20 25 15 20 25 30 composition networkmacromolecular compound (parts by weight) Methanol (parts by weight) 9085 80 75 85 80 75 70 Shearing viscosity of the fuel (dyne · s/cm²) 30008000 25000 85000 120000 180000 230000 290000

COMPARATIVE EXAMPLE 1

50 mL of a 1 wt % sodium alginate methanol solution was poured into a100 mL beaker. 10 g of calcium chloride was placed in a 100 mL beakerand distilled water was added in the beaker to make a volume of 100 mL.The sodium alginate methanol solution was gradually added dropwise tothis aqueous calcium chloride solution by using a syringe while thesolution was stirred by a homogenizer (trade name: Homogenizer PH91,manufactured by MST (Corp.)) at 100 rpm. The produced microparticleswere filtered using a 25 mesh gauze and the particles on the gauze werecollected as a methanol solid fuel (microcapsule fuel).

[Evaluation of the Fuel Cell]

<Fabrication of a Unit Cell>

A platinum-ruthenium catalyst layer and a diffusion layer of carbonpowder-carbon paper were thermally bound to one surface of a polymerelectrolyte membrane in this order under pressure to form an anode (fuelelectrode). The polymer electrolyte membrane was formed from aperfluoroalkylsulfone membrane (trade name: Nafion 112 film,manufactured by Du Pont.). A platinum-ruthenium catalyst layer and adiffusion layer of carbon powder-carbon paper were thermally bound tothe other surface of the perfluoroalkylsulfone membrane in this orderunder pressure to form a cathode (air electrode). With the formations ofthe anode and cathode on the polymer electrolyte membrane, a membraneelectrode having an electrode area of 5 cm² was manufactured.Subsequently, a carbon separator provided with a column flow passage anda current collector were laminated in this order on both surfaces of themembrane electrodes, followed by fastening with bolts to fabricate aunit cell for evaluation.

<Evaluation of the Unit Cell>

Each fuel obtained in Examples 1 to 8 and Comparative Example 1 was fedto the anode side of the fabricated unit cell for evaluation at a flowrate of 7 mL/min and air was fed to the cathode side of the unit cell ata flow rate of 14 mL/min to measure the current-voltage characteristicof the unit cell of 50° C. The results are shown in FIGS. 1 and 2.

As is clear from FIGS. 1 and 2, it is understood that a higher outputvoltage can be drawn from each of the fuels obtained in Examples 1 to 8than from the fuel of Comparative Example 1.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the inventions. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the inventions. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the inventions.

1. A gel-like fuel for a fuel cell comprising: a network macromolecularcompound formed by cross-linking a macromolecular compound comprising atleast one group selected from an OH group and a COGH group, or a C=Nbond with a cross-linking agent, the cross-linking agent consisting ofan isocyanate in which an NCO group is bonded with a long-chain alkylgroup; methanol incorporated into the network macromolecular compound;and a co-catalyst, wherein a shearing viscosity of the fuel is 3000 to290000 dyne·cm².
 2. The gel-like fuel of claim 1, wherein themacromolecular compound is a polyvinyl alcohol.
 3. (canceled)
 4. Thegel-like fuel of claim 1, wherein the macromolecular compound is apolyalginic acid.
 5. The gel-like fuel of claim 1, wherein themacromolecular compound is a polyethyleneimine. 6.-11. (canceled) 12.The gel-like fuel of claim 1, wherein the co-catalyst is porphyrin.13.-15. (canceled)